The present invention relates to a magneto-optical recording medium such as a magneto-optical disk, a magneto-optical tape and a magneto-optical card, and more particularly, it relates to a magneto-optical recording medium capable of magnetically induced super resolution reproduction and a method of manufacturinging a magneto-optical recording medium.
A magneto-optical disk is regarded as a leading memory for storing massive data in the multimedia age ahead, and its recording capacity is desired to further increase. In order to increase the recording density of a magneto-optical disk, it is necessary to make a recording mark length shorter than a spot diameter of a laser beam as well as to reduce the interval between recording marks. It is comparatively easy to form such a small recording mark. However, in reproducing a small recording mark, there is a limit in a reproducible recording mark length because of restriction of the wavelength .lambda. of the laser beam used in the reproduction and the numerical aperture NA of an objective lens.
Therefore, a method of magnetically induced super resolution (MSR) reproduction for reproducing a mark to be reproduce in a laser spot by masking marks except the mark to be reproduce with a initializing magnetic field and a reproducing magnetic field has been proposed, the marks being recorded smaller than a laser spot diameter.
As a countermeasure, the present Applicant has proposed Japanese Patent Application Laid-Open No. 7-244877 (U.S. Pat. No. 5,623,458) as an MSR reproducing method which is an improvement over the above-mentioned method. In this reproducing method, data is read from an intermediate temperature area under application of a reproducing magnetic field of several hundreds Oe, namely, a double mask is formed in a low temperature area and a high temperature area, so as to realize high resolution reproduction without using an initialization magnet. The magneto-optical disk used in this reproducing method includes a reproducing layer, an intermediate layer and a recording layer of rare earth-transition metal alloy. A front mask is formed in a low temperature area utilizing a temperature distribution caused in the laser spot by rotating the medium and irradiating laser beam for reproduction, and a rear mask is formed in a high temperature area. In the intermediate temperature area, the direction of magnetization of the recording layer is transferred onto the reproducing layer.
When a magneto-optical output is detected in the magneto-optical disk, a magneto-optical signal cannot be read from the low temperature area and the high temperature area in a laser spot because of the masks formed therein, but can be read from the intermediate temperature area alone. Accordingly, in the magneto-optical disk proposed by the present Applicant, a recording mark can be read with high resolution from an area substantially narrower than a laser spot under application of a magnetic field of several hundreds Oe in the reproduction without using a large initialization magnet.
The magnetic film such as said reproducing layer, intermediate layer and recording layer is formed by sputtering on the basis of the element composition of the rare earth-transition metal alloys that has been set. However, the magnetic characteristics of the magnetic film vary when the type of film-forming apparatus and the condition under which the film is formed vary, even when the element composition of the film to be formed is fixed. This causes the aforementioned problems, impeding production of the magneto-optical disk with good reproducibility that has a desired magnetic characteristics such as jitter required for the product. Furthermore, even when the film is formed using the same apparatus and under the same condition, the film composition may vary because the target in the apparatus is worn out in the course of successive processing of a great number of magneto-optical disks. This hinders the production of the magneto-optical disk with good reproducibility having various characteristics required for a product.
In a currently used magneto-optical disk having a recording capacity of 640 MB on one side, the track pitch is 1.1 .mu.m, and a minimum mark length of recording marks recorded therein is 0.64 .mu.m. In order to further increase the recording capacity to about of 1.3 GB, for example, the track pitch should be 0.9 .mu.m and a minimum recording mark should have a length of 0.38 .mu.m.
When data of 1.0 GB or more is recorded in and reproduced from the above-mentioned magneto-optical disk capable of MSR reproduction, the track pitch is so narrow that formation of the masks is unstable, and a recording power margin of jitter is reduced depending upon the magnetic characteristics of the recording layer, the intermediate layer and the reproducing layer. Furthermore, although the maximum output of the existing magneto-optical drive is limited to about 300 Oe due to downsizing and power saving, a reproducing magnetic field larger than 300 Oe is required in some cases depending on the magnetic characteristics of the recording layer, the intermediate layer and the reproducing layer. Moreover, when high-density data are repeatedly recorded/reproduced, the quality of reproducing signals is easily degraded and the durability is lowered depending on the magnetic characteristics of the recording layer, the intermediate layer and the reproducing layer.